ABB MPM810 Power module

Description

RTU protocol is a query-response protocol used for communication between the RTU device and a

host computer, which is capable of communicating using RTU protocol. The host computer is the

master device and it transmits a query to a RTU slave, which responds to the master. The RTU slave

device cannot query; it can only respond to the master. A PACSystems CPU can only function as an

RTU slave.

The RTU data transferred consists of 8-bit binary characters with an optional parity bit. No control

characters are added to the data block; however, an error check (Cyclic Redundancy Check) is

included as the final field of each query and response to ensure accurate transmission of data.

Note: You should avoid using station address 1 for any other Modbus slave in a PACSystems control

system because the default station address for the PACSystems CPU is 1. The CPU uses the

default address in two situations:

1. If you power up without a configuration, the default station address of 1 is used.
2. When the Port Mode parameter is set to Message Mode, and Modbus becomes the

protocol in STOP Mode, the station address defaults to 1, unless you specify a STOP Mode

for the serial port in the CPU configuration, and then change the station address to be

used for STOP Mode.

In either of these situations, if you have a slave configured with a station address of 1,

confusion may result when the PACSystems CPU responds to requests intended for that

slave.

The time between the end of a query and the beginning of the response to that query is called the

slave turnaround time. The turnaround time of a PACSystems slave depends on the Controller

Communications Window time and the sweep time of the PACSystems. RTU requests are processed

only in the Controller Communications Window. In Normal sweep mode, the Controller

Communications Window occurs once per sweep. Because the sweep time on PACSystems can be

up to 2.5 seconds, the time to process an RTU request could be up to 2.5 seconds. Another factor is

the Controller Communications Window time allowed in Hardware Configuration. If you configure a

very small Controller Communications Window, the RTU request may not be completed in one sweep,

causing RTU processing to require multiple sweeps. For details on CPU window modes, refer to

Window Modes in Chapter 4.

6.3.1.3 Receive-to-Transmit Delay

Part of the RTU Slave Turnaround time is the receive-to-transmit delay. The RTU driver inserts this

delay after a request from the master has been received, and before the response to the master is

sent. Starting with Release 6.70 for the RX3i, the receive-to-transmit delay can be configured with the

Serial Port Setup COMMREQ function 65520. The timeout is specified in units of 10 ms, with a range of

0–255 units (maximum delay is 2.55 seconds). If the specified time is less than 3.5 character times,

then the delay is set to 3.5 character times.

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